The invention relates to a method for determining the wavelength of light impinging onto the surface of a light sensitive semiconductor structure, and relates furthermore to a color-sensitive sensor circuit comprising such a light sensitive semiconductor structure.
Before explaining the details of the specific method and circuit according to the invention, a number of different light sensitive semiconductor structures, known from various publications, will be discussed to delimit the state of the art.
In the article "Colour imaging system using a single CCD area array", published in IEEE Transactions on Electronic Devices ED-25, Nr. 2, (1978), pages 102-107, a color-sensitive device is described which is in principle a combination of three photo detectors receiving the impinging light through respective optical red, green and blue filters and, applied in a suitable pattern onto the upper side of the sensor surface to realize the actual color filtering. With this construction it is possible to determine the intensity of the corresponding primary or complementary color. From the information provided by the three detectors it is possible to determine the unique position of the color in the CIE color triangle. Large color imaging sensors have already been realized using such techniques. A disadvantage thereof is, however, that for each light sensitive element three diodes are necessary as well as a corresponding color filter for each of them. The relatively complicated construction and the relatively large surface necessary for the construction of an array of such light sensitive elements leads to a rather large drop-out rate during the fabrication thereof.
Further semiconductor structures are proposed in which for each light sensitive element only two diodes are present. The functioning of these known structures is generally based on the fact that the absorption of impinging light into a semiconductor material is largely dependent on the wave-length of the light. Light of short wave-length will be absorbed near the surface of the semiconductor material whereas light of longer wave-length will penetrate deeper into the material before it is absorbed.
This difference in penetration capacity is used in the device described in the U.S. Pat. No. 4,011,016. Therein a light sensitive semiconductor structure is described which in fact comprises two mutually insulated photo diodes at a different level beneath the surface of the semiconductor material. Impinging light of short wave-length will mainly be detected by the diode nearest to the surface, whereas impinging light of longer wave-length will in general be detected by the deeper diode. By comparing the photo currents of both diodes information can be retrieved about the wave-length of the impinging light. The functioning of this known device is based on the fact that different diffusion currents will be generated by illumination with different wave-lengths.
Another semiconductor structure in which use is made of pn-junctions at various depths beneath the light receiving surfae of a semiconductor structure is described in the U.S. Pat. No. 4,107,722.
A disadvantage of these known structures is the fact that still two different pn-junctions for each light sensitive element are necessary, whereas furthermore these two pn-junctions have to be realized at different depths.
A further important disadvantage is that no sharp color filtering is obtained. Only an indication is provided about the average color value, because the color is not excactly reproduced. The response of these prior art sensors on illumination with a combination of red and blue light may be equal to the response on illumination with only green light.